Seisan is a foundation level kata in some Okinawan systems (such as my Isshinryu practice) but in many others it is an intermediate study (such as uechi) or an advanced one such as in Goju Ryu. All these traditions possess similar and different practices.

The most interesting use of the meaning of '13' I've heard goes back to a chinese stylist that claims the embusen of the form (isshinryu's) forms the symbol for '13'. But this is beyond my personal knowledge to explain as I don't do Chinese or Japanese.

Where the kata came from, what it "meant" I cannot know. All I can do with certainty is explore the kata as I understand it currently. If I understand the what & why's of how we demonstrate it... I am far, far ahead of the game.

Why it might be called 13...how can we possibly know? And once "knowing", what fundamental value will that hold? Some strangely contend "13" refers to the number of breaths done during the kata. (in & out =2) Others actually maintain there are 13 opponents defeated. Some propose it is a philosphic designation (ie Buddhist/Taoist) whereby that number has unknown serious importance.

Whatever we believe... will that knowledge help deciper the movements, subtlities, nuances? If not, it is a curiousity but not especially helpful I fear... I wish I had the definative answer for you, but the question leads to madness

Seisan dachi is a forward stance, 60/40 weight on front foot. Feet are about shoulder width and deep enough to leave a fist length gap between you forward heel and rear leg knee if you lower you rear leg knee to the ground. Both feet are pointed to the front.

This is so in both Ryukyu Hon Kenpo/Kobujitsu and Chito Ryu. I don't know about other ryu.

If Seisan is an original Shaolin form, then the number thirteen would be Buddhist symbolism. Buddhism has lots of numbers of things. 13 could be the eight-fold path and the five precepts. I don't know if there's anything that is 13 by itself.